In a wireless communication system having at least one transmitter and at least one receiver, the receiver must acquire the timing of a signal transmitted by the transmitter and synchronize to it before information can be extracted from the received signal. The timing of signals transmitted from a base station, within a wireless communication system, is commonly referred to as the system timing.
In cellular wireless communication systems employing Orthogonal Frequency Division Multiplexing (OFDM), synchronization to the timing of a signal enables the exact positioning of a Fast Fourier transform (FFT) window utilised by a receiver of the signal to extract information from the signal.
In any cellular wireless communication system having multiple base stations (BTS) and multiple mobile communication devices the synchronization process must occur frequently between the BTS and the mobile communication devices for the system to be operable. The mobile communication devices will simply be referred to hereinafter as UE (user equipment).
Furthermore, each BTS defines a geographic transmission region, known commonly as a cell, in which UE in substantially close proximity to a particular BTS will access the wireless communication system. The process whereby a particular UE selects a BTS from which to access the cellular wireless communication system is known as cell selection. In order to optimize the reception of the BTS signal, the UE needs to identify the best quality signal received from different BTSs and switch its receiver to tune into the best BTS for a given time. Thus, due to the mobility of UE, the synchronization process has to be employed frequently in order to allow seamless handoffs from one BTS to another BTS as the UE changes location.
In most current cellular wireless communication systems, fast system access and cell selection are essential functions for proper mobile UE operation. The objective of fast acquisition is to allow UE to synchronize into the desired BTS. The cell selection and re-selection is performed by UE to synchronize and measure the signal (including the interference) power among the adjacent BTS and select and switch to the BTS with the best signal quality, namely the maximum C/I (carrier-to-interference) ratio.
Existing solutions to access a wireless communication system employing OFDM (Orthogonal Frequency Division Multiplexing) were designed for wireless LAN (local area network) systems for fast packet access under a SISO (single input-single output) configuration. However, the wireless LAN does not have the capability to deal with the UE mobility, which requires seamless BTS handoff. On the other hand some cellular systems e.g. 3G UMTS are capable of performing cell selection and BTS identification and BTS C/I ratio measurement.
Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) is a novel highly spectral efficient technology used to transmit high-speed data through radio channels with fast fading both in frequency and in time. For a high-speed downlink packet data transmission system, the design of the physical layer packet structure is a fundamental aspect.
OFDM technology has been adopted by DAB, DVB-T and IEEE 802.11 standards. DAB and DVB-T are used for audio and video territorial broadcasting. In these systems, the signal is transmitted in a continuous data stream. A preamble is not needed because fast packet access is not critical. DAB and DVB-T are also applied in single frequency networks. In this case, every transmitter transmits the same signal as a simulcast. The interference from the neighbouring transmitters can be treated as an active echo, which can be handled by the proper design of the prefix. IEEE 802.11 is the wireless LAN standard. It is a packet based OFDM transmission system. A preamble header is introduced in this standard.
Synchronization within MIMO-OFDM (Multiple Input Multiple Output-OFDM) systems, in which each transmitter and each receiver have multiple antennae, is even more difficult. Adding to the complexity of the task is that a fast synchronization process must be very reliable at very low C/I ratio conditions to allow a high rate of success for the entire cell. In addition, high mobility causes a high Doppler spread and this makes reliable synchronization even more difficult.
In MIMO-OFDM systems, synchronization can be performed in two steps. First, frame synchronization (also called coarse synchronization) is performed in order to determine the approximate range of the location of the starting position of the first OFDM symbol in the frame. Second, timing synchronization (also called fine synchronization) is performed to determine the precise FFT window location, so that demodulation in the frequency domain can be performed accurately.
Conventionally, fine synchronization is implemented in time domain. This is achieved by inserting an a priori known pilot training sequence in the time domain for the receiver to perform the cross correlation computing at select time slots.
For example, as shown in FIGS. 1A and 1B, the OFDM frame structure of the IEEE 802.11 standard utilizes several repeated short OFDM symbols generally indicated at 5 arranged as several headers in the time domain at the beginning of the frame for select sub-carriers, followed by training OFDM symbols 207 for fine synchronization. The headers 5 are used for frame (i.e. coarse) synchronization. The training OFDM symbols 207 are used to position the FFT window precisely so that demodulation in the frequency domain can be performed accurately. The training OFDM symbols 207 are followed by a TPS OFDM symbol 205 and data OFDM symbols 30.
The TPS (transmission parameter signalling) OFDM symbol 205, shown more clearly in the frequency domain (see FIG. 1B), is transmitted with a frequency that corresponds to an adaptive coding and modulation period. The training OFDM symbols, TPS OFDM symbol and data OFDM symbols use all sub-carriers. In the 802.11 system, the repeated headers for coarse synchronization are only transmitted on every fourth sub-carrier. This design is only suitable for a simple SISO OFDM system with only a single transmit antenna. For MIMO-OFDM system the preamble design is more complicated because of the existence of multiple transmit antennas. Furthermore for mobile communications, an efficient preamble design is even more difficult because of the multi-cell environment, the requirement for initial access when no BTS information is available, BTS switching and even soft handoff.
Existing methods in the process of cell acquisition and synchronization employ a 3-step-synchronization approach adopted by UMTS WCDMA system, which requires a relatively long access time. While fine synchronization may be performed in the time domain, the self-interference of MIMO channels limits the performance of this approach under very low C/I conditions. Increasing the length of the correlation can enhance the performance of fine synchronization in the time domain but at the price of an increase in overhead and processing complexity. The existing designs are based on the time domain training sequence correlation for a single transmit antenna and a single receive antenna system. However, a straightforward extension of such a time domain synchronization approach will cause performance loss especially for low C/I ratio applications. The cause of the performance loss is the self-interference between the MIMO channels that is not easy to reduce in time domain.